Methane conversion

ABSTRACT

A method for synthesizing hydrocarbons from a methane source which comprises contacting methane with an oxide of Ru. The oxide is reduced by the contact which is carried at about 500° to 1000° C. Reducible oxides of Ru are regenerated by oxidizing the reduced composition with oxygen.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 06/600,669, filed 4/16/84, now U.S. Pat. No. 4,489,215.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to synthesis of hydrocarbons from a methanesource. A particular application of this invention is a method forconverting natural gas to a more readily transportable material.

2. Description of the Prior Art

A major source of methane is natural gas. Other sources of methane havebeen considered for fuel supply, e.g., the methane present in coaldeposits or formed during mining operations. Relatively small amounts ofmethane are also produced in various petroleum processes.

The composition of natural gas at the wellhead varies but the majorhydrocarbon present is methane. For example, the methane content ofnatural gas may vary within the range from about 40 to about 95 volumepercent. Other constituents of natural gas include ethane, propane,butanes, pentane (and heavier hydrocarbons), hydrogen sulfide, carbondioxide, helium and nitrogen.

Natural gas is classified as dry or wet depending upon the amount ofcondensable hydrocarbons contained in it. Condensable hydrocarbonsgenerally comprise C₃ + hydrocarbons although some ethane may beincluded. Gas conditioning is required to alter the composition ofwellhead gas, processing facilities usually being located in or near theproduction fields. Conventional processing of wellhead natural gasyields processed natural gas containing at least a major amount ofmethane.

Large-scale use of natural gas often requires a sophisticated andextensive pipeline system. Liquefaction has also been employed as atransportation means, but processes for liquefying, transporting, andrevaporizing natural gas are complex, energy-intensive and requireextensive safety precautions. Transport of natural gas has been acontinuing problem in the exploitation of natural gas resources. Itwould be extremely valuable to be able to convert methane (e.g., naturalgas) to more readily handleable or transportable products. Moreover,direct conversion to olefins such as ethylene or propylene would beextremely valuable to the chemical industry.

Recently, it has been discovered that methane may be converted to higherhydrocarbons by a process which comprises contacting methane and anoxidative synthesizing agent at synthesizing conditions (e.g., at atemperature selected within the range from about 500° to about 1000°C.). Oxidative synthesizing agents are compositions having as aprincipal component at least one oxide of at least one metal whichcompositions produce C₂ + hydrocarbon products, co-product water, and acomposition comprising a reduced metal oxide when contacted with methaneat synthesizing conditions. Reducible oxides of several metals have beenidentified which are capable of converting methane to higherhydrocarbons. In particular, oxides of manganese, tin, indium,germanium, lead, antimony and bismuth are most useful. Seecommonly-assigned U.S. Pat. Nos. 4,443,649; 4,444,984; 4,443,648;4,443,645; 4,443,647; 4,443,644 and 4,443,646.

Commonly-assigned U.S. patent application Ser. No. 522,935, filed Aug.12, 1983, discloses and claims a process which comprises contactingmethane with an oxidative synthesizing agent under elevated pressure(e.g., 2-100 atmospheres) to produce greater amounts of C₃ + hydrocarbonproducts. The entire content of this application is incorporated hereinby reference.

Commonly-assigned U.S. Patent Application Ser. No. 522,938, filed Aug.12, 1983, discloses and claims a process for the conversion of methaneto higher hydrocarbons which comprises contacting methane with particlescomprising an oxidative synthesizing agent which particles continuouslyrecirculate between two physically separate zones--a methane contactzone and an oxygen contact zone. The entire content of this applicationis incorporated herein by reference.

Commonly-assigned U.S. Pat. No. 4,499,322 discloses and claims a processfor the conversion of methane to higher hydrocarbons which comprisescontacting methane with an oxidative synthesizing agent containing apromoting amount of alkali metal and/or compounds thereof. The entirecontent of this application is incorporated herein by reference.

Commonly-assigned U.S. Pat. No. 4,495,374 discloses and claims a processfor the conversion of methane to higher hydrocarbons which comprisescontacting methane with an oxidative synthesizing agent containing apromoting amount of alkaline earth metal and/or compounds thereof. Theentire content of this application is incorporated herein by reference.

SUMMARY OF THE INVENTION

It has now been found that methane may be converted to higherhydrocarbon products by contacting a methane-containing gas with a solidcomprising a reducible oxide of ruthenium.

Methane is desirably contacted with the solid at a temperature withinthe range of about 500° to 1000° C. Hydrocarbons produced by the processmay include lower alkanes, lower olefins and aromatics. The rutheniumoxide is reduced by contact with methane and is reoxidizable by contactwith an oxygen-containing gas.

DETAILED DESCRIPTION OF THE INVENTION

Reducible oxides of Ru can be supplied from a variety of known sources.The term "reducible" is used to identify those oxides which are reducedby contact with methane at temperatures within the range of about 500°to 1000° C. Preferred oxide is ruthenium dioxide.

The contact solid employed in the present process may also contain, in apreferred embodiment of the present invention, at least one alkali metalor alkaline earth metal. Alkali metals are preferred. Sodium isparticularly preferred. The amount of alkali/alkaline earth metalincorporated into the contact solid is not narrowly critical. Thepreferred atomic ratio of the reducible ruthenium oxide component(expressed as the metal, Ru) to the alkali/alkaline earth metalcomponent (expressed as the metal, e.g., Na) is within the range ofabout 1-500:1, more preferably within the range of about 2-100:1, stillmore preferably within the range of about 2-10:1.

The contact solid may also contain other components. For example, thesolids may contain components heretofore referred to as oxidativesynthesizing agents. Oxidative synthesizing agents generally comprise atleast one oxide of at least one metal, which oxides when contacted withmethane at synthesizing conditions (e.g., at a temperature selectedwithin the range of about 500° to 1000° C.) produce higher hydrocarbonproducts, co-product water, and a reduced metal oxide. The compositionthus contains at least one reducible oxide of at least one metal. Theterm "reducible" is used to identify those oxides of metals which arereduced by contacting methane at synthesizing conditions (e.g., attemperatures selected within the range of about 500°-1000° C.). The term"oxide(s) of metal(s)" includes: (1) one or more metal oxides (i.e.,compounds described by the general formula M_(x) O_(y) wherein M is ametal and the subscripts x and y designate the relative atomicproportions of metal and oxygen in the composition) and/or (2) one ormore oxygen-containing metal compounds, provided that such oxides andcompounds have the capability of performing to produce higherhydrocarbon products as set forth herein.

Oxidative synthesizing agents have previously been found to comprisereducible oxides of metals selected from the group consisting of Mn, Sn,In, Ge, Sb, Pb, and Bi and mixtures thereof. Particularly effectiveoxidative synthesizing agents have been found to comprise a reducibleoxide of manganese and mixtures of a reducible oxide of manganese withother oxidative synthesizing agents.

It is within the scope of the present invention to include othereffective oxidative synthesizing agent components wit ruthenium oxide.Thus, the ruthenium oxide system may also contain a reducible oxideselected from the group consisting of Mn, Sn, In, Ge, Sb, Pb, Bi andmixtures thereof.

It is also within the scope of the present invention to include at leastone phosphorus component in the solid contacted with methane.

While the exact composition of the contact solids is more complex, apreferred group of solids employed in the process of this invention maybe described by the following empirical expression:

    Ru.sub.a B.sub.b C.sub.c P.sub.d O.sub.e

wherein B is selected from the group consisting of alkali and alkalineearth metals; C is selected from the group consisting of Mn, Sn, In, Ge,Pb, Sb, Bi and mixtures thereof; a, b, c, d and e indicate the atomicratio of each component; and when a is 10, b is within the range ofabout 0.5-10, c is within the range of about 0-10, d is within the rangeof about 0-10, and e has a value which is determined by the valences andproportions of the other elements present.

These components may be associated with other support materials.However, in a presently preferred embodiment, a reducible oxide of Ru isemployed as a support for the other components of the solids. While useof other supports is within the scope of this invention, it has beenfound that the use of bulk ruthenium oxides give superior results.

Accordingly, a particularly preferred embodiment of the presentinvention comprises contacting methane at a temperature within the rangeof about 500° to 1000° C. with a solid comprising a member of the groupconsisting of alkali metals and compounds thereof associated with asupport comprising a reducible oxide of Ru. Preferably, the reducibleoxide of Ru comprises ruthenium dioxide. Still more particularly, thepresently preferred alkali metal associated with these supports is Na.

The contact solids employed in this invention can be prepared by anysuitable method. Conventional methods such as precipitation,co-precipitation, impregnation, or dry-mixing can be used. Supportedsolids may be prepared by methods such as adsorption, impregnation,precipitation, co-precipitation, and dry-mixing. When phosphorus isincorporated in the agent, it is desirable to provide it in the form ofa phosphate of an alkali metal or an alkaline earth metal. Substantiallyany compound of these elements can be employed in the preparation of thepromoted synthesizing agent.

A suitable method of preparation is to impregnate a support withsolutions of compounds of the desired metals. Suitable compounds usefulfor impregnation include the acetates, acetylacetonates, oxides,carbides, carbonates, hydroxides, formates, oxalates, nitrates,phosphates, sulfates, sulfides, tartrates, fluorides, chlorides,bromides, or iodides. After impregnation the preparation is dried toremove solvent and the dried solid is prepared for use by calcining,preferably in air at a temperature selected within the range of about300° to 1200° C. Particular calcination temperatures will vary dependingupon the particular metal compound or compounds employed.

If phosphorus is used, the alkali/alkaline earth metal and phosphorusare preferably added to the composition as compounds containing both pand alkali/alkaline earth metals. Examples are the orthophosphates,metaphosphates, and pyrophosphates of alkali/alkaline earth metals.Pyrophosphates have been found to give desirable results. Sodiumpyrophosphate is particularly preferred.

Regardless of how the components of the contact solid are combined, theresulting composite generally will be dried and calcined at elevatedtemperatures.

The present process is distinguished from previously suggested methaneconversion processes which rely primarily on interactions betweenmethane and at least one of nickel and the noble metals, such asrhodium, palladium, silver, osmium, iridium, platinum and gold. Anexample of this type of process is disclosed in U.S. Pat. No. 4,205,194.The present process does not require that methane be contacted with oneor more of nickel and such noble metals and compounds thereof.

Moreover, in a preferred embodiment, such contacting is carried out inthe substantial absence of catalytically effective nickel and the noblemetals and compounds thereof to minimize the deleterious catalyticeffects of such metals and compounds thereof. For example, at theconditions, e.g., temperatures, useful for the contacting step of thepresent invention, these metals when contacted with methane tend topromote coke formation, and the metal oxides when contacted with methanetend to promote formation of combustion products (CO_(x)) rather thanthe desired hydrocarbons. The term "catalytically effective" is usedherein to identify that quantity of one or more of nickel and the noblemetals and compounds thereof which when present substantially changesthe distribution of products obtained in the contacting step of thisinvention relative to such contacting in the absence of such metals andcompounds thereof.

In addition to methane, the feedstock employed in the method of thisinvention may contain other hydrocarbon or non-hydrocarbon components,although the methane content should typically be within the range ofabout 40 to 100 volume percent, preferably from about 80 to 100 volumepercent, more preferably from about 90 to 100 volume percent.

Operating temperatures for the contacting of methane-containing gas andreducible ruthenium oxide are generally within the range of about 500°to 1000° C. If reducible oxides of metals such as In, Ge or Bi arepresent in the solid, the particular temperature selected may depend, inpart, on the particular reducible metal oxide(s) employed. Thus,reducible oxides of certain metals may require operating temperaturesbelow the upper part of the recited range to minimize sublimation orvolatilization of the metals (or compounds thereof) during methanecontact. Examples are: (1) reducible oxides of indium, (operatingtemperatures will preferably not exceed about 850° C.); (2) reducibleoxides of germanium (operating temperatures will preferably not exceedabout 850° C.); and (3) reducible oxides of bismuth (operatingtemperatures will preferaby not exceed about 850° C.).

Operating pressures for the methane contacting step are not critical tothe presently claimed invention. However, both general system pressureand partial pressure of methane have been found to effect overallresults. Preferred operating pressures are within the range of about 1to 100 atmospheres, more preferably within the range of about 1 to 30atmospheres.

Contacting methane and the solid comprising a reducible ruthenium oxideto form higher hydrocarbons from methane also produces a reduced metaloxide and co-product water. The exact nature of the reduced metal oxidesare unknown, and so are referred to herein as "reduced metal oxides".Regeneration of a reducible metal oxide is readily accomplished bycontacting such reduced materials with oxygen (e.g., anoxygen-containing gas such as air) at elevated temperatures, preferablyat a temperature selected within the range of about 300° to 1200° C.,the particular temperature selected depending on the metal(s) includedin the solid.

In carrying out the present process, a single reactor apparatuscontaining a fixed bed of solids may be used with intermittent or pulsedflow of a first gas comprising methane and a second gas comprisingoxygen (e.g., oxygen, oxygen diluted with an inert gas, or air,preferably air). The methane contacting step and the oxygen contactingstep may also be performed in physically separate zones with solidsrecirculating between the two zones.

Thus, a suitable method for synthesizing hydrocarbons from a methanesource comprises: (a) contacting a gas comprising methane and particlescomprising a reducible Ru oxide to form higher hydrocarbon products,co-product water, and reduced Ru oxide; (b) removing particlescomprising reduced Ru oxide from the first zone and contacting thereduced particles in a second zone with an oxygen-containing gas to formparticles comprising a reducible Ru oxide; and (c) returning theparticles produced in the second zone to the first zone. The steps arepreferably repeated at least periodically, and more preferably the stepsare continuous. In the more preferred embodiment solids are continuouslycirculated between at least one methane-contact zone and at least oneoxygen-contact zone.

Particles comprising a reducible Ru oxide which are contacted withmethane may be maintained as fluidized, ebullating, or entrained beds ofsolids. Preferably methane is contacted with a fluidized bed of solids.

Similarly, particles comprising reduced Ru oxide which are contactedwith oxygen may be maintained as fluidized, ebullating or entrained bedsof solids. Preferably oxygen is contacted with a fluidized bed ofsolids.

In one more preferred embodiment of the present invention, methanefeedstock and particles comprising a promoted oxidative synthesizingagent are continously introduced into a methane contact zone maintainedat synthesizing conditions. Synthesizing conditions include thetemperatures and pressures described above. Gaseous reaction productsfrom the methane contact zone (separated from entrained solids) arefurther processed--e.g., they are passed through a fractionating systemwherein the desired hydrocarbon products are separated from unconvertedmethane and combustion products. Unconverted methane may be recoveredand recycled to the methane contact zone.

Particles comprising reduced metal oxide are contacted with oxygen in anoxygen contact zone for a time sufficient to oxidize at least a portionof the reduced oxide to produce a reducible metal oxide and to remove,i.e., combust, at least a portion of any carbonaceous deposit which mayform on the particles in the methane contact zone. The conditions of theoxygen contact zone will preferably include a temperature selectedwithin the range of about 300° to 1200° C., pressures of up to about 30atmospheres, and average particle contact time within the range of about1 to 120 minutes. Sufficient oxygen is preferably provided to oxidizeall reduced metal oxide to produce a reducible oxide and to completelycombust any carbonaceous deposit material deposited on the particles. Atleast a portion of the particles comprising promoted oxidativesynthesizing agent which are produced in the oxygen contact zone arereturned to the methane contact zone.

The rate of solids withdrawal from the methane contact zone is desirablybalanced with the rate of solids passing from the oxygen contact zone tothe methane contact zone so as to maintain a substantially constantinventory of particles in the methane contact zone, thereby enablingsteady state operation of the synthesizing system.

In one alternative process employing the method of this invention, a gascomprising oxygen may be co-fed with a hydrocarbon gas comprisingmethane to the methane contact zone. See U.S. patent application Ser.No. 06/600,656, the entire content of which is incorporated herein byreference.

In a further alternative process employing the method of this invention,the olefin content of the effluent produced by methane conversion asdescribed herein may be oligomerized to produce normally liquid higherhydrocarbon products. See U.S. patent application Ser. No. 06/600,657,the entire content of which is incorporated herein by reference.

In a still further alternative process employing the method of thisinvention, it has been found advantageous to recover C₂ + alkanes from(1) the effluent produced by methane conversion as described hereinand/or (2) streams derived from such effluent and to recycle suchalkanes to the methane contact zone. See U.S. patent application Ser.No. 06/600,878, the entire content of which is incorporated herein byreference.

In a still further alternative process employing the method of thisinvention, it has been found that halogen promoters enhance resultsobtained when methane is converted to higher hydrocarbons by contactwith a reducible metal oxide. See U.S. patent application Ser. No.06/600,668, the entire content of which is incorporated herein byreference. Also see U.S. patent application Ser. Nos. 06/600,659(chalcogen promoters) and 06/600,658 (NO_(x) promoters), the entirecontents of which are incorporated herein by reference.

What is claimed is:
 1. A method for converting methane to higherhydrocarbon products which comprises contacting at a temperature withinthe range of about 500°-1000° C. a gas comprising methane and a solidcomprising a reducible oxide of Ru.
 2. The method of claim 1 wherein thegas comprising methane contains from about 40 to about 100 volumepercent methane.
 3. The method of claim 1 wherein the gas comprisingmethane contains from about 80 to about 100 volume percent methane. 4.The method of claim 1 wherein the gas comprising methane contains fromabout 90 to about 100 volume percent methane.
 5. The method of claim 1wherein the gas comprising methane is natural gas.
 6. The method ofclaim 1 wherein the gas comprising methane is processed natural gas. 7.The method of claim 1 wherein the reducible oxide is ruthenium dioxide.8. A method for synthesizing hydrocarbons from a methane source whichcomprises:(a) contacting at a temperature within the range of about500°-1000° C. a gas comprising methane and a solid comprising areducible oxide of Ru, said contacting producing C₂ + hydrocarbons,coproduct water and solids comprising a reduced Ru oxide; (b) recoveringC₂ + hydrocarbons; (c) at least periodically contacting the solidscomprising reduced Ru oxide with an oxygen-containing gas to produce asolid comprising a reducible Ru oxide; and (d) contacting a gascomprising methane with the solids produced in step (c) as recited instep (a).
 9. The method of claim 8 wherein the temperature of step (c)is within the range of about 300° to 1200° C.